Mechanism behind the Unusually High Conductivities of High Concentrated Sodium Ion Glyme-based Electrolytes

Abstract

Highly concentrated electrolytes based on ether solvents for sodium electrochemical cells are studied. The investigated electrolytes use sodium triflate and glycol diethers oligomers of different length to form the electrolyte. These electrolytes present conductivities that increase as a function of concentration even when the electrolyte is composed of a majority of ion pairs and aggregates. Correlation analysis between the electrolyte speciation and conductivity suggests the presence of two distinct mechanisms of charge transport, namely a traditional vehicular mechanism based on the diffusion of free ions and a hoping mechanism involving the making and breaking of ion-pairs and/or aggregates. The former mechanism represents the charge transport of glyme with 3 or 4 units, while the latter is observed in electrolytes composed of short chains; i.e., 1 or 2 units. The proposed mechanism of transport is corroborated via molecular dynamics simulations. In addition, our experiments demonstrate that the high concentration of the sodium salt not only increases the overall conductivity of the electrolyte, but also does not affect its electrochemical window.